"The human brain is not prepared to understand this – even on the steps of the gas chamber" … Claude Lanzmann. Photograph: TODD HEISLER/New York Times / Redux / eyevine

Claude Lanzmann went to Iran recently. "As you know," the 85-year-old director, a Jewish Frenchman, tells me in his Paris office, "Ahmadinejad doesn't believe there was a Holocaust. The Iranians wanted me to prove to them on television that there was. They wanted to see the corpses."

What did he tell them? The director of the nine-and-a-half hour documentary Shoah (1985) about the mass murder of Jews in Nazi death camps swivels round in his chair and fixes me. "I told them there's not a single corpse in Shoah. The people who arrived at Treblinka, Belzec or Sobibor were killed within two or three hours and their corpses burned. The proof is not the corpses; the proof is the absence of corpses. There were special details who gathered the dust and threw it into the wind or into the rivers. Nothing of them remained."

Among the Jews detailed to dispose of human remains was Simon Srebnik, whom Lanzmann lured from his home in Israel to the site of Chelmno, the first camp where Jews were gassed. In Shoah's opening sequence, we see Srebnik being rowed along the Narew river. As the boat eases through calm waters, Srebnik sings, his lovely voice mingling with the sound of the breeze in the summer trees.

"It is not beautiful," snaps Lanzmann when I tell him my first impression of this sequence. Only later do we learn that what Srebnik is singing is a Nazi marching song that, during his captivity, he was taught and compelled to sing for his captors' entertainment. Only later do we learn that Srebnik was one of the Jews compelled by Nazis to daily dump sacks of crushed bones of Holocaust victims into this all-too-calm river. Two days before Chelmno was liberated by Soviet troops, remaining prisoners were shot in the head, among them Srebnik. Amazingly, he survived.

Shoah, which will be screened as part of the London documentary film festival Open City later this month, followed by a Q and A with the director, is a documentary of absences. There is no newsreel footage, there are no old photos, no corpses. Sometimes Lanzmann trains his camera on an empty field for several minutes. We see a seeming bucolic idyll – just the place for a picnic. Only the caption – Treblinka – tells us something intolerable happened here.

For a long time, Lanzmann tells me, he resisted going to Poland. "Why would I want to? What would I see?" Instead, he toured the world interviewing Holocaust survivors for his film, pushing them hard to recall their experiences. Interviewees such as Abraham Bomba, whom Lanzmann filmed cutting hair in his Tel Aviv salon. As Bomba worked, he told Lanzmann how he was forced to cut women's hair at Treblinka just before they were gassed.

At one point in the interview, Bomba recalled how a fellow barber was working when his wife and sister came into the gas chamber. Bomba broke down and pleaded with Lanzmann that he be allowed to stop telling the story. Lanzmann said: "You have to do it. I know it's very hard." This was his principal method on Shoah: to incarnate the truth of what happened through survivors' testimonies, even at the cost of reopening old wounds.

With testimonies such as these, Lanzmann initially thought, he needn't go to the scene of the crimes – to death camps such as Treblinka, Belzec, Sobibor or Auschwitz-Birkenau. But, four years into his work on Shoah, Lanzmann changed his mind. "Finally, I realised I was meeting people, but couldn't understand what they were telling me. I had to go there. I arrived in Poland loaded like a bomb with knowledge. But the fuse was missing – Poland was the fuse."

What astounded him when he arrived in villages near the death camps was that life carried on regardless – as though the tragedy of the Holocaust had been erased. "When I saw the village of Treblinka still existed, that people who were witnesses to everything still existed, that there was a normal train station, the bomb that I was exploded. I started to shoot."

What he started to shoot were testimonies of non-Jewish Polish bystanders. Were they oblivious to what was happening? Overwhelmingly not: Lanzmann interviews Jewish victims and bystanders who recalled that non-Jewish Poles made throat-cutting gestures to Jews as they arrived at the death camps on trains – to alert them to what was about to happen, perhaps, or maybe to revel in their looming murders. Lanzmann found evidence of Polish antisemitism in the villages around the death camps: a male interviewee relates how he's happy the Jews are gone, but would rather they had gone to Israel voluntarily than be exterminated. In an interview outside a Catholic church, with Simon Srebnik present, bystanders alleged the Holocaust was just retribution for the killing of Jesus.

While inculpating Poles in Shoah, Lanzmann in this interview exculpates the Allies from the charge of doing nothing to save the Jews. "Could the Jews have been saved? My answer is no. I'm very deeply convinced of this. Everybody talks about the bombing of Birkenau. Some in the War Refugee Board [created by President Franklin Roosevelt in 1943] were for bombing, and there were others who were against for reasons that cannot be despised." What reasons? "Some pilots asked, 'What is the meaning of this, to bomb the people we're meant to rescue?' A terrible contradiction.

"Money, not bombs, would have helped the Jews, because the Germans were running out of money. But in wartime you can't send money because there are rules. But some religious Jews did send money to Slovakia that got into German hands, and for a while the deportations stopped."

The question of whether the allies could have saved the lives of the Jews goes to the heart of one of the most important interviews Lanzmann conducted for Shoah, namely the one with the Polish spy and diplomat Jan Karski. In 1943, Karski was commissioned by the exiled Polish government to tell allied leaders about the fate of Poland, and by two Jewish leaders in Warsaw to do the same about the fate of the Jews. "They asked him to mobilise the conscience of the world," says Lanzmann. In Shoah, Karski recounts what he saw in the ghetto and in camps. At the end of that interview, Karski says of his visit to Washington and London: "I made my report."

Why end the interview there? "Everybody knows that the Jews were not rescued. He didn't need to say more. It was very strong to end that way."

But last year, Lanzmann changed his mind. He decided to release a film of the rest of the 33-year-old Karski interview, in which he told Lanzmann in detail of his mission to brief allied leaders. In this new film, The Karski Report, the Polish spy tells us that he met Supreme Court Justice Felix Frankfurter, a Jew, who, upon hearing Karski's description of the horrors befalling Jews in Poland, said: "I do not believe you." But Frankfurter was not calling Karski a liar. Indeed, at the same meeting, Frankfurter clarified what he meant: "I did not say that he was lying, I said that I could not believe him. There is a difference."

Human inability to believe in the intolerable is what The Karski Report is about. At the start of the film, Lanzmann quotes the French philosopher Raymond Aron, who, when asked about the Holocaust, said: "I knew, but I didn't believe it, and because I didn't believe it, I didn't know." No wonder Lanzmann, a friend of Jean-Paul Sartre and lover of Simone de Beauvoir, is concerned with such philosophical issues. "The human brain is not prepared to understand this – even on the steps of the gas chamber. Karski says this very clearly." Hence, for Lanzmann, the primacy of oral testimony as a mode of representation and understanding at the heart of Shoah.

But that primacy is paradoxical: the tragedy of Karski's mission, if it was a tragedy, was to have witnessed something of such unprecedented horror that no mere report could convey its import, still less move the allies to action.

Why release this film now? Lanzmann released The Karski Report after the publication in 2009 of a novel called Jan Karski by the French writer Yannick Haenel. The novel became a French bestseller, but Lanzmann attacked it as "a falsification of history and of its protagonists". "It's a scandal about Karski, because he tries to make Karski into a man obsessed with the rescue of the Jews. He was not." So Karski was not, as Haenel's novel implies, the man who tried and failed to stop the Holocaust? "No! He says: 'The Jews were not the centre of my mission. Poland was the centre of my mission.' He says that very clearly."

"I said to myself, 'You are an idiot, because you have the film of the second day's interview to show that Karski was not as he is depicted in this novel.' So I released The Karski Report to re-establish the truth." Haenel, for his part, argues Lanzmann does not understand his novel.

But what is the truth? Is truth only what emerges from oral testimony such as that given by Shoah's interviewees? Sometimes, just as Adorno injuncted writing poetry after Auschwitz, so Lanzmann seems to be prohibiting – or at least reserving the right to slur – art about the Holocaust that is not based on oral testimony. Isn't something to be said for artists who, in an act of creative empathy, try to imagine the lives of others embroiled in the Holocaust and legacy (consider, say, Nicole Kraus's recent novel Great House, steeped as it is in creatively imagining the lives of Holocausts survivors)? "Of course one can make art about the Holocaust after my film," Lanzmann says. "All I do say is that great literature always adds to reality."

The implication is clear: Haenel's literary imagining of Karski's inner world distorts and subtracts from reality, while Lanzmann clearly believes Shoah, does otherwise. He wrote in the French newspaper Libération recently that when one watches Shoah, "one bears witness for nine hours 30 minutes to the incarnation of the truth, the contrary of the sanitisation of historical science."

"That," he says, "is why it remains important to see my film."

• Shoah and The Karski Report are both being screened at the Open City festival, on 18 and 19 June. Details: opencitylondon.com